The invention is in the field of directly obtaining electrical signals representative of images or of desired qualities of images by using the interaction of images and strain waves to modulate electrical properties of bodies. More specifically, the invention is directed to two-dimensional Fourier imaging using a technique called acoustic pseudo-beam steering, in which two intersecting strain waves are controlled such that they produce the desired effects of a third, pseudo-wave, which would not in fact exist.
The patent specifications referenced above describe a family of DEFT devices and techniques for certain types of electronic imaging and certain uses of such imaging. Such imaging employs coupling between strain waves, an image, and electric properties to produce electrical signals representing components of the spatial Fourier transform of the image, or representing some other desired characteristic of the image. The signals can be used for pattern recognition, all-electronic image magnification and stabilization, motion and velocity detection, focusing and the like. The electrical signals may be reconverted back to an image, if desired, using similar techniques. The patent specifications referenced above have been mainly directed to obtaining one-dimensional transforms of one-dimensional or two-dimensional images. Certain devices and techniques for achieving full two-dimensional capability have been disclosed, but there is still a need for some effective two-dimensional Fourier imaging.
Accordingly, one aspect of the invention is concerned with a new type of two-dimensional Fourier imaging using acoustic pseudo-beam steering. In such two-dimensional imaging two intersecting acoustic waves propagate along the surface of a substrate or substrates and have individually controllable frequencies which are varied in a manner producing the effects of a combined acoustic wave whose direction of travel with respect to the image can be controlled (or steered) as desired and whose frequency characteristics can be similarly controlled. No such combined acoustic wave would in fact be produced; hence the name "acoustic pseudo-beam steering" is used in this specification. This pseudo-beam steering permits the direct derivation of components of the two-dimensional spatial Fourier transform of a two-dimensional image without the difficulties attendant to producing an actual combined wave having desired travel direction and frequency characteristics.
In one specific embodiment of the invention, two surface acoustic waves traveling perpendicularly to each other (or intersecting at some other angle) propagate along a substrate having a thin photoconductive film acoustically coupled to a surface thereof. A two-dimensional image is formed on the film, and the photoconductivity of the film is measured by detecting the current i(t) across the film portions between interdigital electrical contacts formed on the film. This current i(t) is proportional to: EQU i(t) .varies..intg..intg.dx dy E.sub.z.sup.2 (x,y,t) I (x,y) (1)
where I(x,y) is the two-dimensional light intensity distribution of the image in the image plane of the film, x and y are the coordinates in the image plane of the film, t is time, and E.sub.z (x,y,t) is the electric field resulting in the substrate (e.g. a piezoelectric substrate) by the propagation of two surface acoustic waves along the x- and y-directions and is in a direction perpendicular to the image plane. Since EQU E.sub.z = E.sub.1 cos (.omega..sub.1 t - k.sub.1 x) + E.sub.2 cos (.omega..sub.2 t + k.sub.2 y) (2)
where E.sub.1 and E.sub.2 refer to electrical fields resulting in the substrate from strain waves, .omega. is angular frequency of a surface acoustic wave, k = .omega./c, where c is the propagation speed of the wave, E.sub.1, .omega..sub.1, k.sub.1 refer to the surface acoustic wave in the x-direction, and E.sub.2, .omega..sub.2 and k.sub.2 refer to the surface acoustic wave in the y-direction. The square E.sub.z.sup.2 of the electric field E.sub.z (x,y,t) perpendicular to the image plane of the film contains a term of the form: EQU E.sub.z.sup.2 .varies. E.sub.1 E.sub.2 cos[.omega..sub.1 - .omega..sub.2)t-(k.sub.1 x + k.sub.2 y)] = E.sub.1 E.sub.2 cos[(.omega..sub.1 - .omega..sub.2)t-k.multidot.r] (3)
where k = k.sub.x x + k.sub.y y is the vector sum of the wavevectors of the two surface acoustic waves and r = xx + yy is a vector defining a point in the image plane (x and y are unit vectors in the x- and y-direction). By varying the respective frequencies of the two surface acoustic waves traveling in the x- and y-directions, the quantity k can be correspondingly varied, yielding a current signal term i.sub.s (t) proportional to the two-dimensional spatial Fourier transform of the image I(x,y) at the image plane: EQU i.sub.s (t) .varies. exp [j(.omega..sub.1 - .omega..sub.2)t] .intg. d.sup.2 r I(r) exp (-jk.multidot.r) (4)
where I (r) is the intensity distribution of the image at the image plane, while the other terms from expression (1) can be ignored. The measured current i.sub.s (t) thus behaves as if a new, third surface acoustic wave had been created with wavevector equal to the sum of the wavevectors of the two actual surface acoustic waves.
In another specific structure embodiment of the invention, a two-dimensional image is directed along an optical axis transverse to a surface of an elasto-birefringent substrate flanked along the optical axis by two crossed polarizers. The term "elasto-birefringent" designates a material which may or may not be birefringent at rest (preferably is not), but becomes birefringent when excited by strain waves in it. Two crossed acoustic waves having individually controlled frequencies propagate along the substrate, and the light emerging from the combination of the substrate and the polarizers is collected by a measuring device providing a photocurrent signal at the difference frequency of the two acoustic waves. This photocurrent signal contains a component proportional to the k Fourier component of I( r), where k = k.sub.x x + k.sub.y y is the vector sum created by the squaring effect of the birefringence, and I( r) is the light intensity distribution of the image directed to the substrate surface along which the two surface waves propagate. A similar effect may be obtained by using two such substrates, one for a strain wave in the x-direction and another for a strain wave in the y-direction.
The invention therefore relates to forming an image on a selected plane of a body of a material having desired properties, propagating two intersecting or crossed strain waves along or close to that plane, and individually controlling the characteristics of the two strain waves so as to derive from the body electrical signals having characteristics determined by the combination of the characteristics of the two strain waves and having electrical parameters representative of the two-dimensional spatial Fourier transform characteristics of the image. In one specific embodiment of the invention, the image is directed along an optical axis and the image plane is a surface of an elasto-birefringent substrate along which the acoustic waves propagate and which is flanked along the optical axis by two crossed polarizers such that the light emerging along the optical axis can be measured to derive said electrical signals, while in another specific embodiment of the invention the image is formed on an electrophotoconductive film acoustically coupled to a surface of substrate along which the acoustic waves propagate and the electrical signals are derived by measuring the photoconductivity of the film. While the principles of these two specific embodiments are similar, each has certain advantages and disadvantages with respect to the other. Additionally, the invention encompasses similar modulation of the junction of two materials of opposite electrical properties, such as a PN junction, at or close to the image plane.